With almost everyone on Atlantis infected by an amnesia-causing virus, McKay (David Hewlett) finds himself zap-strapped to his desk with cryptic letters on his arm and no memories of who he is or what has happened.

SG-1 searches for an ancient weapon which could help them defeat the Ori, and discover it may be in the Ori's own home galaxy. As the Ori prepare to send ships through to the Milky Way to attack Earth, SG-1 travels to the Ori galaxy aboard the Odyssey. The International Oversight committee has their own plans and SG-1 finds themselves in a distant galaxy fighting two powerful enemies.